Division by unvalidated freq<=0 -> kernel divide-by-zero panic

Summary

_systimer_init_periodic(:269/:271/:273) and systimer_adjust_periodic(:347) pass freq to fromhz() without checking freq>0. fromhz=cputimer_freq/freq+1 (kern_cputimer.c:199). freq=0 -> div-by-zero panic. No in-tree caller passes 0 today.